Use different toctrees for top-level sections

When merging the different sections of our documentation (devel,
sysadmin and user) a single general toctree was defined for navigation.

Using a single toctree definition for all top-level sections result in
a sidebar with only these sections, fully collapsed, which feels
bad for discoverability and quick navigation.

Instead we can define a toctree for each section, each one with their
specific caption. This reduces the depth of the navigation tree by
one-level, while providing a better overview.

The downside of this change is that we duplicate the toctree for each
section. They would need to be kept in sync to avoid surprises when
performing independent builds.